Textile control



May 2G, 1952 w. c. ANDERSON 2,5%973 TEXTILE CONTROL Filed July l5, 1948 2 SHEETS-smi 1 OSCILLAOR RELHY AMPLIFIER l www www 10 INVENTOR W/LMER CZ ANDERSON May 20, l952 w. c.. ANDERSON 2,596,973

TEXTILE CONTROL Filed July 13., 1948 2 Sl-EETS--SHEET 2 T0 OTHER PHIRS OF COILS QN LEFT SIDE. OF' CREEL TO OTHER PAIRS OF COILS ON RIGHT-SIDE `gaa/OF CREEL.

dei m 665 408+ 201s n H95 i, l

N VE N TO R W/L MER CH/voE/fo/y NEY Patented May 20, 1952 UNITED STATES PATENT ortica TEXTILE CONTROL Wilmer -C. Anderson, Greenwich, Conn., assignor toDeering Milliken Research Trust, New York, N. Y., a nonprofit trust f New York Application July 13, 1948,'seria1No. 38,390

(c1. ca -51) 12 Claims. 1

The present invention relates to the Vcontrol of tex-tile .rnachinery, and more particularly to :methods Aandmeans 'for detecting yarn breakage, "or .reduction in yar-n tension, in arrays of yarn "ends or threads moving 'under tension, as forex- -ample thewa'rp ends of' a Vwarper or of a loom.

There Fare various devices, called stop motions, n'ow in use onwarp'ers which Ioperate upon break- 'age of a warp end V.to l'stop the motor driving the -takel-'up beam or otherwise vto arrest the travel of 'the warp. `Inrrro'stof thedevices now in commercial `Suse, each end of yarn, destined to form 'fthe warp of a fabric, .passes through an eye of a fdrop wire andsupports the wire out of contact withfalmeta'l bar below it. When the end breaks, the l-resulti'ng slackening 'of tension therein perimits vthe drop wire to make contact 'with the vb'ar, making fan electrical connection with a bar sholding the drop wires and closing a control circuit for '-Ia-rr'esting I'rotation of the take-up beam.

A -very 'corn'rnon cause of faulty operation in suc-h i-.prior art devices is the tact that the electrical contact arrangement iis open and is thus exposed to dirt -and lint and to the corrosive fefect of moisture, any lor all of which may prevent proper contact -of the drop wire with the 4bai below it y'and may thus inhibit the flow of Vsuiiicient current in the -control circuit. When Vrelatively high voltages are 'used at thecontacts,

there is danger "of shock to the operator. Also with :high voltages dust particles, and y, accumulate by electro-'static attraction.

Another 'disadvantage :in prior art vpractice is the flack 'vof any indication 'to the operator 'as to' fwhich fend thas broken. There are normally -several 'hundred ends gathered in the warper, `'and lit is `impossible for the operator standing in 'a position-'behind the beamto determine which 'of `these-ends has broken. :A skilled-operator can very often sight the broken end on la single trip clown the length of one side of the creel, but this Ican "be done only when he has 'the good fortu'ne to chance upon the side which has the 4brokenend; 'f if he picks the wrong side, he must "retracehis steps, go 'around the warper and start -all the way up the length of the other side. It will :be apparent :that a great deal of time is thus wasted, during most ot which time the warper -couldbe in operation if indication of the position of the broken end were provided.

The use of a circuit including an indicator "lig-ht for this purpose and actuated by the drop wire contact .has Ybeen suggested, and lsuch -cir- "cui-ts may be 'set lup 'foreachend for each of v.azm'imb'er'of banks'of 'endsyorfor each side of fthe creel. The diiculty in such an yarrangement 'wouldbe that the operator would have .to keep his eyes x'edon the indicator Ypanel at all times to determine which light went on first. The stopping of the beam .immediately thereafter causes laXity or slackness in'every end, and all the drop wires promptly fall, which would complete all the light circuits.

.An object of the invention is to provide a method of detecting yarn breakage which does knot utilize circuit closing elements carried by the yarn and hence may be 'practised with apparatus not subject to failure when exposed to lint, dirt, moisture'or the like.

A further object of the invention :is to provide a stop motion for warpers which is sensitive and reliable in operation and which is free 'from hazard of shock to the operator. p 4

The `invention also has in Acontemplation the provisionfof an improved stop motion unit for use in a warping operation wherein an indicator system promptly'designates the location of a broken Y end.

The present invention further provides an improved circuit for a stop motion device in association with a warper, which circuit includes means for limiting designation of a broken end to `only one of a .plurality of positions.

Other objectsand advantages of this invention will be apparent from the following description, taken in conjunction with the accompanying drawings, wherein:

Fig. 1 is a circuit diagram explanatory of the I invention;

Fig. 2 is .a schematic' perspective view of a stop motion device .'for use in association with a warper and vincludes a diagram Aof the electrical circuit employed;

Fig. 3 is a Vfragmentary top plan view, partly broken away, and Yon an -enlarged scale, of a pair of coils forming .part 'of the stop ymotion device Yo"ffl'ig.v"2 .and showing the orientation of the coils apparatus provided for practising the method of the invention, a small element of relatively high magnetic permeability is suspended from an end of yarn traveling under tension and a coil carrying alternating current creates an alternating magnetic eld in the vicinity of the element so that, when the end breaks, the reduction in tension therein permits the element to enter the field and to change the ux linked with the coil. The change in inductance of the coil is detected and used for creation of a control current for signalizing yarn breakage, preferably by arresting the travel of the yarn, as hereinafter described. Thus the invention completely obviates the use of contact closing drop wires with their attendant susceptibility to failure from accumulation of lint and their inherent hazard of shock to the operator when high voltages are employed.

In Fig. 1, to which reference may now be had, there is diagrammatically illustrated a circuit arrangement suitable for use in practicing the invention. For simplicity, the system of Fig. 1 is shown as arranged for detection of breakage of a single end of yarn. It will be understood that the yarn, indicated at 2, is being advanced under tension in the direction of the arrow by any suitable means (not shown). The yarn passes through suitable guide means, indicated at 4 and 6, which determine its path of travel. An element 8 of relatively high permeability is freely suspended from the yarn 2 at one point of its travel. Any suitable means, such, for example, as hereinafter described in connection with Figs. 3 to 5, can be provided for preventing travel of the element with the yarn. In Fig. 1 such means are diagrammatically indicated as comprising a fixed stop I0.

A pair of matched inductance coils, I2 and I4, are connected in series across the secondary winding of a transformer I6, the primary Winding of which is supplied with alternating current by an oscillator I8. Coil I2 is so positioned with respect to the element 8 that the element will be introduced into the core of the coil when the tension in the yarn is reduced, as by breakage. An amplier 20 is connected across the midpoint of the secondary of the transformer I6 and the junction ofthe coils I2 and I4. With the above described arrangement and with substantial initial balance of the circuit, a relatively large signal will be applied to the amplifier when the balance of the circuit is upset by the introduction of element 8 into core I2. This signal, after amplification, is applied to a relay 22. Relay 22 may control a signal device, such as an indicating lamp or may control the circuit of the yarn advancing means or may perform both of these functions as desired and as well known in the art.

VAlthough the detecting circuit of Fig. 1 has been described with reference to the detection of breakage of a single end of yarn, the principle of the system is equally applicable to detection of breakage of any one or more of an array of yarn ends. A plurality of pairs of series-connected matched coils can be provided, one pair for each end, with all detecting coils connected in parallel and all matching coils connected in parallel, or a single large balancing coil can be provided for balancing a plurality of parallel-connected detecting coils. Preferably, however, the detecting coil is so dimensioned as to accommodate in its core a plurality of elements 8, and a matching coil is provided for each detecting coil and positioned adjacent thereto so as to be subject to the same conditions of temperature and humidity. Such preferred arrangement is embodied in the warper stop motion device illustrated in Figs. 2 to 5, to which reference may now be had.

In Fig. 2, a textile wai-per is indicated diagrammatically as comprising a creel 24, carrying cones of yarn 26, a comb 28, a warp beam 30 and drive motor 32 for the warp beam. Rotation of the warp beam 3i! by the motor 32 draws the ends from the cones 26 through the comb 28 and Winds them upon the Warp beam. In the drawing, the array 34 of yarn ends represent those ends drawn from cones on the right and left side of but part only of the creel. Similar arrays of yarn ends from higher and lower banks of the creel will be drawn through the comb 28 to the warp beam. Parts of such other arrays are indicated at 36. In the embodiment of the invention illustrated in Fig. 2 a pair of of coils I2a and I4a, corresponding to coils I2 and I4 of the circuit of Fig. l, is associated with those ends of array 34 coming from cones on the right side of the creel and another such pair of coils, indicated at |222 and I4b, is associated with the ends of array 34 coming from cones on the left side of the creel. Coil IZa is connected in parallel with other detecting coils (not shown) for other ends from the right side of the creel and coil |401J is connected in parallel with balancing coils (not shown) associated with such other detecting coils, as indicated by the legend applied at lines 38a. Similarly the coils I2b and I 4b are respectively connected in parallel with other detecting and balancing coils (not shown) for the left side of the creel as indicated by the legend applied to lines 38D. Each detecting and balancing coil, as shown best in Figs. 3 to 5, is composed of a plurality of turns of Wire wound on a long shallow hollow core 40 of Bakelite or the like. The coils of each pair are mounted vin a framework 42, which is suitably supported at its ends from any fixed part of the system. The upper part of each frame 42 comprises a strip 44 of right angle stock having a vertical wall section 45 and an upper horizontal wallsection 46. The wall section 45 is provided with a plurality of eyelets 48 for passage of the ends therethrough and the upper section 46 is provided with a like number of eyelets 49 through leach of which extends 'a wire 50. Each wire 50 has a looped yarn guide eye 50' by means of which the wire is supported on the yarn. A metal strip 8 of ferromagnetic material, such as iron or an alloy thereof, is welded or otherwise secured to each Wire 5I] below the loop 50' therein. Each wire 50 beyond the part at which the strip is secured, is bent at right angles to prevent the strip fromv falling through the core of the detecting coil when the associated end of yarn is slacked and to provide a means for lifting the strip during threading of the warper. The detecting coil is so mounted in the framework 42 as to have its core positioned beneath the strips 8. The balancing coil is mounted alongside the detecting coil so as to be subject to like conditions of temperature and humidity, but is positioned so as to be uninfiuenced by dropping of an element 8 as a result of yarn breakage.

The coils I2a and I4a are connected in series with a resistor 54a across the secondarywinding of the transformer I6, the primary of which is supplied with alternating current from the oscillator I8. The junction ofthe coils I2a and' I4a 'motor 432.

15 -is connected A"through a winding 56a with Van 'adjustable ntap on fa resistor '58a also Vconnected `vacross thesecondary winding ofthe transformer.

r-p'loyed -inforder vto provide a fail safe arrangement 'land also to providean 'interlockfor giving an indication, 'as jbetween the two -sides of the Warner, of Ythe location #of yarn breakage.

'Asshown'linllig. 2,-the ungrounded outputfterlrninal .of amplier 20a is `connected through a capacity `62a fand resistor AElia 'with the vcontrol lgrid of *a triode flia vand is lalso 'connected ito ground through 'a series-connected capacity 68a vand resistor 10u. The vcontrol grid oi.' tube 66a is connected to ground 'through la pairrof seriesfconnected 'resistors 12a and 74a, of which refsistor "14a, is `'substantially larger vand is bridged by -a capacity `16a. A rectifier 18a lhas `its anode connected to 2the junction of resistors 12a and 14a and it'scathodefconnected 'to the ungrounded end ol resistor 1150a. The fanode of triode Esa-'is connected through Ia resistor 80a to 'a lsuitable source A'of positive potential, vindicated as B+.

This anode is also connected to the control fgrid of -a lsecond triodeil-Za -through a condenser 84a.

The cathode fof the -triode 82a is operated at posi- `Vtive potential, as indicated, and the anode circuit o'f this tube includes the Winding of a control relay V86a 'having two armatures 88a and 89a `associated therewtih. Thus, with an initial slight unbalance of the 'circuit including coils I-2a and 14a, a "signal amplified by triode '66a and fsu'iiicient to maintain triode 82a energized 'is obtained Ybut, vwhen a relatively Alarge signal is As the Vchannel lassociated with coils 12b vand Mb .is shown as `identical with that just described, andas 'the elements thereof are identiii'ed 'with like reference numerals, except 'that 'the 'subscript vb 'is lattached thereto, no `additional ,Y

descriptionthereofisnecessary.

Control .relay `86u, 'and the corresponding vcontrol 'relay'''b of the circuit associated with coils 12b yand v'lwlbjointly control, over their armatures 88a 'and Mb and associated front contacts, the lcircuit of a holding relay V9i! for `the circuit of vWhen both "relays 86a and 86o are energized, and a manually operable switch, indicated vat S, lis closed, relay 90 is energized from a suitable source of `energy (not shown) 'con-V 'nectedto terminals 92. When energized, relay 90 f holds the circuit of motor 32 closed over an armature y9,4 and front contact associated therewith.

'Deenergizationfof either relay v86a or y861) opens the circuit of the holding 'relay which relay, 1inv turn, opens the circuit of motor 32 to arrest vrotation of the warp ybeam 30.

In 'order that an "indication Amay be given'as to the 'location o'f the yarn break Ninitiating deenergization of 'the nro'tor 132, a Vpair of signal `devices, Vim'lic'ated yat R `and L, are provided. TheV Ycircuit-foi deviceifltlrmaybe traced as lfollows: 'from vlone 'terminal "S2, lthrough leads 96, 98 and 'I'UU to -|the1indicator`R,lead `I`02a to a back contact `lliaassociated with armature d8a, armature jMaj-lead 5F06, iront Contact Vi @8b associated with 'armaturefb'oi relay 861), armaturelband lead '-II-l back -to the other terminal 92. The circuit fof'indicator L includes a back `contact 1041) asso- Yciated'with armature 88h, varmature 88h and .lead I l0. Thus, when both relays I86d and 86h are energized, thefcircuits of both indicators are open. -When relay 86a is deenergized as vthe result of -breakage 'of a Ayarnend on vthe right 'side ofthe `Creel, Athe circuit of indicator R is closed atback contact 11'04a. lSimilarly, deenergzationof relay -lbcau'ses'closure of the circuit of indicator L.

-When 4"the warp beam '30 ceases to rotate, all

ends are r`slac'ke'cl Aand consequently, with the circuit lso f'ar described, both relays 86a and 86h nwould "be eventually deenergized each time an 'endbreaks and the circuit of indicator R would open -at contact 1138i). In order to avoid this eventuality,-circuit means are provided for maintaining leither vcontrol relay energized upon deenerg'iza'tion of the other. For this purpose a YVback contact v4112e associated with armature Sila is grounded, and Iarmature 89a is connected by a 'lead Hdb to the junctionof resistors 12b and 14h. Similarly back contact l I2b associated with armature :891D is grounded and that armature of relay 8627i is `connected 4by a lead Hdd with the junction of resistors 72a 'and 14a. Thus, when 'relay 86a is 'de'energized the bias voltage is removed irom'th'e -control grid of tube 66h and consequentlythat triode'remains energized irrespective of the application of a signal from lamplifier 210e. Triode B2b and'r'elay 86h thus remain energized 'and signal R is energized to 'indicate Vthe division ofthe creel in whichgyarn breakage has occurred. When jyarn. breakage occurs in the left side of the creel, relay 86h is deenergized, signal `L is energized and .relay 86a is maintained energized by removal of the bias applied to the control grid of tube 66a.

The operation of vthe system of Fig. 2 will be apparent from the description already given. As the control relays 86a and 86h and the holding relay 90 are normally energized, any failure of the circuit 'will result in stoppage of the warp beam. Thus there Iis no danger of false safe indcations. The unbalance voltage suincient to maintain .the control vrelays energized can be readily adjusted by adjustment of the position o'f the taps onresistors`58a and 58h of the balance circuits and by adjustment of other circuit elements, for example resistors 54a and 54h, as will be apparent to those .skilled in the art. By the provision voli the indicators with their interlock- Ying control, positive indication of the location of yarn breakage is given.

As the .cores of Vdetecting coils 12a and 12b are open ended, lint or dust :cannot accumulate therein. Moreover, as no electrical, or even physical, contact of parts in the neighborhood of the yarn is required rfor detection of breakage, the system is not subject to the defects Ainherent in tprior art systems. The elements can be light `in weight and small in size and can be of a variety of materials. `Satisfactory operation has been lobtained 'when the elements were of an alloy of iron `'and nickel and had a thickness of .014 inch, a width of .375 inch and a length of 2.25 inches. With suitably dimensioned detecting and balancing-coils, a relatively large signal can'be obtained with the circuit o'f Fig. 2 when a yarn suspended element enters the core of a detecting coil. For

example, inone specific construction, the detecting coils had an inductance of 25 millihenrys each, were made of 315 turns of #28 wire and had a major core dimension of about 20 inches so as to accommodate 30 drop elements.V With such construction a ratio of signal voltage to unbalance voltage of 30 to 1 at the input to the amplifier was obtained, when the oscillator irequency was kilocycles.

The invention has now been described with particular reference to a specific embodiment thereof. Obviously, the broad principle of the invention, the detection of yarn breakage or yarnlaxity of one or of a plurality of ends by detection of change of magnetic induction in a, medium, can be practised with devices and with circuits other than those specificaly illustrated and described and various changes can be made in the particular stop motion device of the drawings without departing from the spirit of the invention or from the scope of the appended claims. The coil, the current through which creates the alternating magnetic field, and the inductance of which changes upon introduction into its core of a yarn carried element, need not necessarily be an element of a bridge network, as changes in inductance of the coil can be detected without recourse to balancing networks. Balancing networks are preferred, however, because of the relatively high sensitivity obtained therewith. Various other balancing circuits, as heretofore suggested, can be employed in lieu of the specific circuit illustrated. At present the plurality of matching pairs of adjacent coils is deemed to be preferred because, with such arrangement, localized changes in temperature or humidity do not aect the balance of the circuit. An alternative arrangement and one which halves the number of coils required for any given number of ends of yarn is one in which both coils of each pair are positioned to serve as detecting coils so that introduction of an element into the core of either coil of a pair upsets the balance of the circuit associated therewith. This is a satisfactory arrangement as the likelihood of simultaneous introduction of elements into both coils of a pair is relatively remote. Although the fail safe type of circuit shown in Fig. 2 is preferred, obviously the system can be arranged to energize, rather than to deenergize, controlling relays. In the speciiic system illustrated in Fig. 2, but two indicating channels have been shown. Additional channels with interlocking indicators can be provided, if indications of yarn breakage at more than two sections of the creel are desired. Other modifications within the scope of the invention will be apparent to those skilled in the art.

The stop motion device of the invention has been described with particular reference to a textile warper, but obviously the principle thereof can be employed in a stop motion device for any machine in which one or a plurality of yarn ends are advanced under tension. For example, the invention can be used in twisters and spoolers, 0r in looms.

In the appended claims the term yarn is intended to include any textile lamentary material, whether twisted or untwisted and whether composed of continuous laments or of staple bers.

The following is claimed:

1. In a textile machine having means for moving a plurality of endsof yarn under tension, a

stop motion device for the yarn advancing means comprising in combination means for creating an alternating magnetic field beneath a portion of the normal path of the ends, a plurality of elements of magnetic permeability greater than air, one suspended from each yarn end above the eld so as to beintroduced therein upon reduction in tension of a supporting end, and circuit means responsive to the introduction of an element into the eld for stopping said yarn advancing means, said circuit means including a part the effective impedance of which is altered when an element is introduced into the ield.

2. In a textile machine having means for advancing a plurality of yarn ends under tension, a stop motion device for the yarn advancing means comprising in combination a circuit including at least one air core coil carrying alternating current and positioned beneath a portion of the path of travel of at least some of the ends, a plurality of elements of relatively high magnetic permeability suspended from the yarn ends, each element being suspended from a different end and at least one of said elements being suspended above said coil so as tobe introduced into the core thereof upon reduction of tension in the supporting end and means responsive to a change in the inductance of the coil occasioned by introduction of an element into the core for stopping the yarn advancingV means.

3. The stop motion device according to claim 2 wherein said circuit is al bridge network having said coil in one branch thereof and being normally substantially in balance, said last mentioned means being responsive to substantial unbalance of the network occasioned by change in the permeability of the core of said coil.

4. The stop motion device according to claim 2 wherein a plurality-of said elements are suspended above said coil whereby said last mentioned means operates to stop the yarn advancing means when the tension in any one of the ends supporting said elements is reduced.

5. In a textile machine having means for advancing a plurality of ends of yarn under tension, a stop motion device for the yarn advancing means comprising in combination a plurality of inductors positioned adjacent the path of travel of the ends, a plurality of elements each associated with a different end and adapted, upon reduction in tension of the associated end,` to be introduced into the core of an adjacent inductor and means responsive to a change in the permeability of the core of any of the inductors for stopping said yarn advancing means.

6. The combination according to claim 5 including at least two indicating devices, circuit means for causing operation of one of said devices When the permeability of the core of any inductor in one group of inductors changes and for causing operation of the other of said devices when the permeability of the core of any of the inductors in anothergroup of inductors changes and interlocking means effective when one of said devices operates to prevent operation of the other of said devices.

'7. In a textile machinehaving means for advancing a plurality of ends of yarn under tension, the combination comprising a source of electrical oscillations, a pair of substantially Imatching coils, a circuit connecting said coils in series across said source, a second circuit so coupled to said rst circuit as to detect unbalance in the effective impedance of said coils, a plurality of elements of relatively high permeability suspended from a group of yarn ends, means for supporting one of said coils beneath said elements with the core of the coil positioned for introduction therein of an element when its supporting yarn end is slacked, and relay means controlled by said detecting circuit for stopping said yarn advancing means upon a predetermined unbalance of the effective impedance of said coils corresponding to that resulting from introduction of one of said elements in the core of the coil positioned thereunder.

8. The combination according to claim 7 including another pair of substantially matching coils, a circuit connecting said coils in series across said source, a second detecting circuit so coupled to said last mentioned circuit as to detect unbalance in the eifective impedance of said last mentioned pair of coils, a plurality of other elements of relatively high permeability suspended from a second group of yarn ends, means for supporting one of said last mentioned coils beneath said last mentioned elements with the core of the coil positioned for introduction therein of an element when its supporting yarn end is slacked, second relay means controlled by said last mentioned detecting circuit for stopping said yarn advancing means upon a predetermined unbalance of the effective impedance of said last mentioned pair of coils, a pair of indicators each operatively connected to a different one of said relays and selectively responsive to the operation of its associated relay, and interlocking means interconnecting said relays and detecting circuits to prevent operation of one of said relays after the other has operated as a result of yarn' break, whereby said advancing means is automatically stopped whenever one of the ends breaks and whereby the group in Which it is included is automatically indicated by one of said indicators.

9. A stop motion for a textile warping machine having a warp beam, the rotation of which advances a plurality of ends of yarn under tension, comprising yarn supported elements of relatively high magnetic permeability, an air core coil adjacent the moving ends with its core positioned for introduction therein of at least one of said elements upon reduction of tension of its supporting yarn end, means for energizing said coil with alternating current, a normally Aenergized electronic device having a control electrode, circuit means connected to said electrode and responsive to the inductance of said coil for causing deenergization of said device upon a predetermined increase in the inductance of said coil corresponding to the introduction of one of said elements into the core thereof, and means controlled by said device and operative upon deenergization of said device for stopping rotation of the warp beam.

10. The stop motion according to claim 9 including a second coil adjacent the moving ends of yarn with its core positioned for introduction therein of at least one other of said elements upon reduction of tension of its supporting yarn end, means for energizing said second coil with alternating current, a second normally energized electronic device having a control electrode, a second circuit means connected to the control electrode of said second electronic device and responsive to the inductance of said second coil for causing deenergization of said second device upon a predetermined increase in the inductance of said second coil corresponding to the introduction of one of said elements into the core thereof, and means controlled by said second device and operative upon deenergization of said second device for stopping rotation of the warp beam.

11. The stop motion according to claim 10 including a pair of interlocking circuits, each connected to the control electrode of one device and controlled by the other device, for preventing deenergization of one device after deenergization of the other device and a separate signal associated with each device for indicating deenergizaticn of the associated device whereby when a yarn end breaks causing an element to enter the core of one of said coils, one of said electronic devices is deenergized, the rotation of the warp beam is stopped and the appropriate signal indicates into which coil an element has been introduced as a result of yarn breakage.

12. The combination with a textile machine having means for advancing textile material, of a source of electrical oscillations, a pair of substantially matching coils, a circuit connecting said coils in series across said source, a second circuit so coupled to said first circuit as to detect unbalance in the eiective impedance of said coils, means for supporting at least one of said coils adjacent textile material being advanced by said advancing means, and relay means controlled by said detecting circuit for stopping said material advancing means When an abrupt change in a characteristic of the material passing said last mentioned coil produces a predetermined unbalance of the eective impedance of said coils.

WILMER -C. ANDERSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,032,176 Kovalsky Feb. 25, 1936 2,116,586 Stoller May 10, 1938 2,390,799 Lisenmeyer Dec. 11, 1945 2,405,218 Lambach Aug. 6, 1946 

